Precipitation, electrostatic explanation

As ionic strength, in Figure 2.3, is increased, the solution again reaches a point where the solute molecules begin to separate from solvent and preferentially form self-interactions among themselves that result in crystals or precipitate. The explanation for this salting-out phenomenon is that the salt ions and macromolecules compete for the attention of solvent molecules, that is, water. Both the salt ions and the protein molecules require hydration layers to maintain their solubility. When competition between ions and proteins becomes sufficiently intense, the protein molecules begin to self-associate in order to satisfy, by intermolecular interactions, their electrostatic requirements. Thus dehydration, or the elimination and perturbation of solvent layers around protein molecules, induces insolubility. 

Anionic Surfactants onto Kaolinite and lUite. In the investigation of the adsorption of sodium dodecylbenzenesulfonate (SDBS) and sodium dodecyl sulfate (SDS) onto asphalt covered kaolinite and illite surfaces, Siffert et al. [5S] observed Langmuir type I isotherms for SDS adsorption onto Na kaolinite and Na illite while the SDBS exhibited a maximum in adsorption with a decrease beginning near the CMC. Adsorption maxima were observed near the CMC for both surfactants in the Ca kaolinite and Ca illite systems. The adsorption behavior was explained as precipitation of the calcium salt of the surfactants (an idea supported by other studies), and the interaction of the aromatic ring in SDBS with the asphalt. This interaction favors desorption of the asphalt rather than adsorption of the SDBS. The amount of asphalt desorbed by SDBS was twice that desorbed by SDS. Other explanations for adsorption maxima include mixed micelle formation [55] and electrostatic repulsion of micelles from the bdayer covered surface [59]. 

The theory states that once sufficient surfactant is available to stabilize primary particles, N should not thereafter depend on SDS concentration. Reference to Figures 6 and 7 shows that this is essentially so with the PBI-initiated systems, but that N continues to increase with [SDS] for an appreciable extent in the Fenton s system. A possible explanation for this difference in behavior between the two may be that the uncharged HO-terminated oligomeric radicals associate with surfactant molecules at higher SDS concentrations even before they precipitate out. This would introduce electrostatic repulsions between oligomer and polymer particle which would reduce the rate, R, of capture. This oligomer-surfactant association would be expected to be more pronounced in the Fenton s Reagent-initiated systems than in PBI systems. 

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